The first hole was for a fan. This one is for some ducting to vent outside.

I think the wall is about 11 inches including plaster. Thinking about it (based upon a 4" hole I drilled), the blocks are probably hollow. The wall is a bit patchworky (with some brickwork among the blocks) so it could be anything.

The first hole was for a fan. This one is for some ducting to vent outside.

I think the wall is about 11 inches including plaster. Thinking about it (based upon a 4" hole I drilled), the blocks are probably hollow. The wall is a bit patchworky (with some brickwork among the blocks) so it could be anything.

Julian F. G. W.

BS EN 771-3 hollow blocks have a standard face size of 440 x 215mm thus your 500 mm hole will be over 2 blocks so chisel out along the pointing and then radius.

That is a big hole. First thing to do would be to check the position isn't going to cause structural problems. If it's in a structural wall I'd make sure it's not close to another opening or point of bearing of a lintel or steel beam. From other posts you seem to have a good idea on these things but I think a caution is worthwhile in casef anyone else reads this at some stage in the future

If it is safe to make the opening I think the easiest way is to take the plaster bach to the blockwork and see what you are up against. If it is pure blockwork angle grinding a full block out and half of the two blocks above or below as appropriate gets you most of the way there. It creates a lot of dust so the other alternative would be to drill a hole in the wall and then cit the blocks out with a special blade in an alligator saw or similar. It's worth a try although some blockwork saws easier than others. It will still create dust but it's not as fine and easier to tidy up.

If the hole is likely to cause structural problems you might need a structural surveyor to have a look at it.

sg31 wrote:Take the plaster bach to the blockwork and see what you are up against. If it is pure blockwork angle grinding a full block out and half of the two blocks above or below as appropriate gets you most of the way there. It creates a lot of dust so the other alternative would be to drill a hole in the wall and then cit the blocks out with a special blade in an alligator saw or similar

Seconded. Use a big (230mm disk) angle-grinder with a diamond blade - and with great care in case you hit steel: the blade will either go straight through that as well, which might have structural consequences, or kick back, which might have painful consequences.(I have a big Makita 9000 series grinder that will go straight through just about anything!)

Removing a block first may work, I'm not sure how the hole will line up with the blocks yet. I don't have an alligator saw but these blades for a reciprocating saw get good reviews, https://www.screwfix.com/p/dewalt-dt242 ... 57mm/2443v although the job might take some time.

I found a Hilti DD350 for hire at £220 for one day. The 500mm core drill would be extra. Unfortunately, there is nothing of significant value the other side of the wall that would justify the cost.